In conventional plastic blow molding machines, the molten plastic is injected into the cavity of an injection mold to apply a parison to a core rod in the cavity. The plasticizer injects the plastic at high pressure and then maintains a reduced pressure which supplies plastic to the mold to compensate for such shrinkage as occurs from the initial cooling of the plastic in the mold cavity.
The plasticizer then draws back plastic to disconnect the runners from the parison and following this operation, the plasticizer screw works its way back in the screw barrel to accumulate a charge of plastic for the next injection operation.
In some apparatus, new core rods can be brought to the injection mold more rapidly than the plasticizer can go through its cycle; and this means that the operation of the plasticizer delays the injection operation and thus limits what could otherwise be higher production of the molding machine.
This invention utilizes two plasticizers which are used alternately. They are operated so that one of the plasticizers can prepare itself for the next injection operation while the other plasticizer is injecting molten material into the injection mold cavity. In order to make this alternate operation practical, this invention utilizes a plastic pump to perform some of the operations of each plasticizer so that after an injection of plastic into the injection mold cavity, the plasticizer which performs the injection can start recovering for the next injection.
As soon as either plasticizer has injected plastic into the injection mold cavity, valve means shut off that plasticizer from communication with the injection mold. The plastic pump maintains a pressure on the plastic to compensate for such shrinkage as occurs immediately after injection; and the plastic pump also draws back the plastic to disconnect the plastic in the runners from the parison in the cavity, both of these functions being ordinarily performed by the plasticizer. Thus the plastic pump permits the plasticizer to start its recovery operation immediately following its injection of plastic into the injection mold.
The second plasticizer stands ready to inject plastic into the injection mold as soon as the next core rod or core rods are in position and the mold has closed. The plastic from the second plasticizer is injected into the mold and the second plasticizer is immediately shut off by a valve from communication with the mold so that the plastic pump can again take over and perform the operations of compensating for shrinkage and drawing back plastic from the mold runners.
The same plastic pump can perform the shrinkage compensation and drawback operation for both of the plasticizers and in the preferred construction the plastic pump always remains in communication with the mold runners of the injection molds so that the only valving necessary is that required for putting the plasticizers alternately in communication with the injection mold followed by immediate cutoff from communication with the injection mold.
A plastic pump used as a time saver in connection with a single plasticizer is disclosed in U.S. Pat. No. 3,709,644, issued Jan. 9, 1973.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.